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PYROMETRY                                            451
pressed off the scale an amount equivalent to 0.26 amp. The lowest scale reading is accordingly 0.26 and the entire scale from 0.26 to 0.60 amp. is thus utilized for the range of the pyrometer lamp.
General Use of Optical Pyrometers.—Optical pyrometers and radiation pyrometers, described later, afford the only means yet developed for measuring temperature above 1,500°C. The high temperature scale above 1,500°C. is based on the extrapolation of Wien's radiation law by means of a pyrometer of the Leeds & Northrup type. When the instrument is especially designed for precision work it is possible to measure a temperature difference of 0.2°C. at 1,500°C. The commercial form of the instrument when properly calibrated can be relied upon to 5°C. With a well-designed optical pyrometer there is a perfect color match of the two fields at all times. Hence, contrary to the general impression, color is not matched at all but simply brightness of uniform color. A color-blind observer will obtain the same settings as a normal observer. Forsythe1 has compiled data obtained with an optical pyrometer of the Leeds & Northrup type by six observers none of whom had ever used an optical pyrometer before. The average variation from the mean for the six inexperienced observers was 3°C. and the maximum variation, 5°C.
Although the optical pyrometer is essential for the measurement of temperatures above 1,500°C. its usefulness is by no means confined to the high temperature range. The thermocouple cannot be adapted to many processes at low temperatures for example, the measurement of the temperature of steel rails as they pass through the rolls, ingots and forgings in the open and small sources such as a heated wire or lamp filament. The temperatures used in the above processes may be accurately measured by the optical pyrometer. The temperature of a microscopic sample of any material can be measured by a modified form of the Leeds & Northrup pyrometer.2 Also in many processes a thermocouple is not so convenient to use as an optical pyrometer, especially when the temperature is not required often enough to warrant a permanent installation of thermocouples.
One serious objection to the optical pyrometer from the industrial point of view is the fact that it has not been made automatically recording. Since a photometric match is required for every setting the instrument necessitates the attention of an observer, although a satisfactory automatic device will be developed eventually. Another objection is the introduction of the human element into the readings, thus affording an opportunity for dishonest or prejudiced settings. The observer, if he is the operator of the furnace, should be taught that the instrument is for his own assistance and that it is not to be considered as a policial measure. Otherwise the measurements should be made by a disinterested party. In a plant operating several furnaces an intelligent boy can be profitably employed whose sole work is to make the rounds of the various furnaces and measure and record the temperatures.
Black-body and Non-black-body Conditions.—Optical pyrometers are usually calibrated to read correctly when sighted on a black body. Many furnaces approximate black-body conditions very satisfactorily. In a perfect black body the details of the inside of the furnace vanish and a piece of steel, for example, which is being heated cannot be distinguished from the back ground. If the
1 Gen. Elec. Rev., Sept., 1904, p. 753.
2 BUEGESS, Bureau of Standards Sci. Paper No. 198.